So, a friend and I decided to build a Tesla Coil. When we started, I wasn’t very familiar with how they worked, but I had plans for a 12″ spark Tesla Coil in Bob Iannini’s book, “Electronic Gadgets for the Evil Genius” Ch. 14, which promised a working Tesla Coil if we followed the plans.
However, I wanted to learn about how Tesla Coils worked from this project. So I started researching Tesla Coils, and I want these blog posts to serve as a compilation of my research and design considerations as I fabricate a Tesla coil.
How do Tesla Coils Work?
The first question I asked before we started this project was: How do Tesla Coils work?
Tesla Coils make high voltage at high frequencies.
Tesla Coils are generally a two stage transformer, taking in the low voltage 60 Hz wall power, and converting it to millions of Volts at somewhere between 200 KHz to 4 GHz.
What does the most basic circuit look like?
How does a Tesla coil change the input frequency?
From a signals perspective, the first question I would have after reading the description would be, “how does the frequency change from 60 Hz to hundreds of kilohertz, or where does the non-linearity of this system come from?” (because linear systems cannot shift the frequency)
Transformers, Capacitors, and Inductors cannot change the frequency, it’s the spark gap.
In the primary side of a Tesla Coil, the spark gap introduces a non-linearity into the system.
There are three major pieces to the general Tesla Coil, as you can see in the above picture. The low voltage input from the wall to just past the high voltage transformer (yellow), the primary from the tank capacitor to the primary coil (green), and the secondary including the secondary coil and the toroidal top (red). The input charges the tank capacitor to a relatively high voltage (thousands of volts) still at 60 Hz until a spark gap breaks down. When this spark breaks down, it becomes a near zero resistance short circuit, which puts an impulse on the primary coil and causes the whole primary circuit to oscillate at it’s natural frequency, a function of the inductance of the primary coil, and the capacitance of the tank capacitor. The RLC (Resistor, Inductor, and Capacitor) circuit at resonance, creates a standing voltage wave that
You can read more about RLC circuits on Wikipedia here. The spark gap is the key to the non-linearity in this system.
How does a Tesla coil make millions of Volts?
The voltage increases from thousands of Volts to Millions of Volts because of resonance between the primary and secondary parts of the circuit.
The secondary side is another RLC circuit. The L is the secondary inductor, and the C is the torus on top. The R comes from the resistance of the windings on the coil. This low series self-resistance from the coil gives the secondary a high quality factor. The complete circuit occurs when air breaks down around the torus and current flows to the surrounding environment. The secondary is designed to naturally oscillate at the same frequency as the primary. Because they oscillate at the same frequencies, the voltage multiplies from the primary to the secondary. This is called a loosely coupled transformer.
Is the primary/secondary a normal transformer?
No. Because the windings of the primary are so far away from the windings of the secondary, and the core of the transformer is air, it is considered a loosely coupled transformer. This means than the ratio of the windings of the secondary to the primary will not be exactly equivalent to the voltage gain. The voltage gain or ratio of secondary voltage to primary voltage is more affected by how well the primary and secondary resonate together, which happens when they are tuned to the same natural frequency.
Are there any other factors that affect the performance?
The secondary should have a length corresponding to the your design frequencies wavelength divided by 4, an antennae and transmission line rule. This is because on the secondary at resonance, a standing voltage wave is created on the length of the wire wrappings of the secondary coil. This standing wave will have a wavelength corresponding to the frequency it is resonating at, which ideally will be 4 times the length of the secondary coil wire.
where f is the frequency of oscillation, c is the speed of light (3E8m/s)
Note that the voltage maximum is where we want our length of wire to be, and it occurs at 1/4 of the width of the sine wave or the wavelength divided by 4.
Practical Applications
What does all of this mean for actually building a Tesla Coil?
This means that you should pick a frequency, I picked 600 kHz, for your Tesla Coil to operate at. Then you should calculate the wavelength, by taking the speed of light 3E8m/s divided by your frequency. Then you should divide this wavelength by 4 to get the length of your secondary coil wire in meters. My length was 125 meters. If you are using PVC to wrap your secondary around, pick a standard size outer diameter of PVC, (mine was 3″ PVC with an outer diameter of 3.5″) and calculate the number of turns by dividing your ideal wire length by pi*outer diameter in meters. 125/(.0889*pi) = 447.56 turns. If you want to be precise, you can also take into account the radius of the wire you are wrapping the PVC with, (mine was 26 gauge). Round up and you now know enough to make your coil.
When winding this coil, the turns should be close together, there should be no overlapping, and no kinks. When you are finished, you can measure the inductance of your coil, and compare to what this website says when you put in your coil dimensions.
In the next post, I will discuss the other half of the secondary, the torus on top, and how to calculate it’s dimensions so that your secondary oscillates at the design frequency.
What can I read to gain a better understanding?
Some other recommended Wikipedia readings are:
